Nickel-chromium-iron alloy and heat treating the alloy



United States Patent 3,459,539 NICKEL-CHROMIUM-IRON ALLOY AND HEATTREATING THE ALLOY Herbert L. Eiselstein and Thomas H. Bassford,Huntington, W. Va., assignors to The International Nickel Company, Inc.,New York, N.Y., a corporation of Delaware No Drawing. Filed Feb. 15,1966, Ser. No. 527,490 Int. Cl. C22c 39/20; C21d 1/26', 1/60 U.S. Cl.75128 8 Claims ABSTRACT OF THE DISCLOSURE The invention is directed toan alloy containing about 29% to about 40% nickel, about 19% to about25% chromium, about 0.2% to about 0.5% carbon, about 0.25% to about1.25% titanium, up to about 1% aluminum, and the balance essentiallyiron. The alloy may be prepared by air melting and has high creep andrupture properties when heat treated at temperatures of about 2300" F.to about 2350 F. for at least about two hours.

The present invention is directed to nickel-chromiumiron alloys and,more particularly, to special nickelchromium-iron alloys havingcontrolled composition and having high creep and rupture strength whileat the same time being relatively inexpensive as compared to knownalloys having a comparable high-temperature strength capability.

The production of alloys resistant to the effects of elevatedtemperatures, e.g., 1400 F. or 1600 F. to 2000 F., or higher, whilebeing subjected to stress has long been a concern of the metallurgicalart. In applications such as piping and other structural forms for usein power and petrochemical plants (e.g., ethylene furnaces) manydifferent alloys are employed. These alloys usually are of thenickel-chromium and nickel-chromium-iron types, With other elementsbeing employed for special purposes. Thus, elements such as cobalt,tungsten, molybdenum, columbinm, aluminum, titanium, etc., are employedto contribute strength, precipitation-hardening capability, oxidationresistance, etc., to the alloys. Certain of the alloying elements whichare commonly employed in heat-resistant alloys are expensive inthemselves and are subject, from time to time, to being available onlyin limited supply. Many of the commonly used alloys, including, forexample, the HK and HOM stainless steels, cannot be produced in wroughtform, such as tubing, and are thus only available in cast form,including centrifugal castings. Furthermore, many of the commonly usedalloys, particularly the less expensive types, becomes embrittled duringlong-time exposure to the combined effects of stress and temperature. Inaddition, many of the commonly used alloys are difiicult to weld, whileothers must be vacuum melted thereby further raising cost.

The art has accordingly been faced with a long-standing problem; to wit,that of providing a relatively inexpensive alloy which would bestructurally stable when subjected to the combined eifects of stress andtemperature, which would have good resistance to creep and rupture atelevated temperatures, which could be produced in the form of largeingots by air melting and air casting techniques, and which couldreadily be converted from the ingot stage to usual wrought forms such astubing, plate, sheet, rod, bar, etc., by usual mill techniques.

We have now discovered a nickel-chromium-iron alloy capable of beingproduced in large ingots by air melting and air casting techniques,which ingots can be commercially converted into common wrought formswhich in use have outstanding resistance to grain growth andembrittlement during mill processing and long-time exposure to elevatedtemperatures and have good stress-rupture properties, while beingrelatively inexpensive.

It is an object of the present invention to provide anickel-chromium-iron alloy having high resistance to creep and rupture.

It is a further object of the invention to provide a nickelchromium-ironalloy having high resistance to grain growth when heated to temperaturesnear the melting point.

It is another object of the invention to provide a nickelchromium-ironalloy having the capability of being produced in wrought form.

Another object of the invention is to provide a nickelchromium-ironalloy which is relatively immune to embrittling effects when exposed tostress at elevated temperature while at the same time having highresistance to creep and rupture.

A further object of the invention is to provide a heat treatment processwhich contributes high rupture strength to the alloy contemplated inaccordance with the invention.

Other objects and advantages of the invention will become apparent fromthe following description.

Broadly stated, the present invention is directed to a creepandrupture-resistant nickel-chromium-iron alloy containing, in weightpercent, about 29% to about 40% nickel, about 19% to about chromium,about 0.2%

- to about 0.5% carbon, about 0.25% to about 1.25%

titanium, up to about 1% aluminum, up to about 0.75% silicon, up toabout 1.5% manganese, and the balance, including small amounts ofincidental elements and impurities not exceeding about 3 beingessentially iron.

Advantageously, the alloys contemplated in accordance with the inventioncontain about 30% to about 35% nickel, abput 10% to about 23% chromium,about 0.35% to about 0.75 titanium, about 0.2% to about 0.5% carbon, andthe balance essentially iron. The advantageous alloy compositionsdisplay a rupture life of at least about hours at 1600 F. and 12,000pounds per square inch (p.s.i.) and, in many cases, a rupture life ofabout 200 hours or more under these conditions. A particularlyadvantageous alloy contains about 20% chromium, about 30% nickel, about0.4% carbon, about 0.5% titanium, and the balance essentially iron.

In preparing the alloy, the chromium and nickel contents are controlledin interrelated amounts in order to maintain satisfactory scalingresistance and creep-rupture resistance in the alloy. Thus, nickel is atleast about 29% and chromium is at least 19% in order to maintainscaling resistance but nickel does not exceed 40% and chromium does notexceed 25 to maintain creep-rupture strength. Carbon is a highlyimportant element in the alloy in order to obtain the desired carbidedispersion-hardening therein. Carbon is at least 0.2% to obtain therequisite strength in combination with the other ingredients in thealloy but does not exceed about 0.5 as otherwise the requisitemalleability in big ingots, such as ingots having a section size ofabout 20 inches square weighing about 8,750 pounds and slab ingotshaving a section of about 17 inches by 55 inches weighing about 17,300pounds. Titanium is another highly important alloying ingredient and itis controlled within the range of about 0.25% to about 1.25% to provide,in combination with the other alloying ingredients, the requisitedispersion strengthening of the alloy. More advantageously, titanium iscontrolled within the range of about 0.35% to about 0.75% or about 0.9%or about 1%. Control of titanium and of carbon in combination isparticularly important in order to permit obtaining the requisitecreep-rupture properties in the alloy. In air melting techniques asapplied to the alloy, an aluminum addition to the molten alloy prior tothe titanium addition performs the useful effect of protecting thetitanium addition from untoward effects, such as oxidation and the like,which could cause unwanted and/or undesirable results. Accordingly, anamount of aluminum of up to about 1% resulting from the aforementionedaluminum addition can be present in the alloy with useful results.Silicon may be present in the alloy in amounts up to about 0.75% withoutencountering harmful effects on the malleability or weldability of thealloy. Those skilled in the art will appreciate that silicon frequentlyforms a constituent of nickel alloy scrap of the kind which can beemployed usefully in melting the alloy. Manganese similarly is found inscrap materials which may usefully be employed in melting the alloy andmay be present therein in amounts up to as much as about 1.5% withoutharmful effect. Columbium, molybdenum and tungsten may also be found inscrap materials, such as mill revert scrap, employed to prepare thealloy. These elements are unnecessary for the production of the specialproperties developed in the alloy but may be present in amounts up toabout 1% each. The impurities sulfur and phosphorus should be presentonly in limited amounts, e.g., in amounts not exceeding 0.015% each and,preferably, in amounts not exceeding about 0.007% each.

countered. To the extent that incipient melting is avoided, theannealing temperature may exceed 2350 F. We have observed incipientmelting in the alloy after heating to 2400 F. for two hours. The dataobtained in creeprupture testing of the alloy indicate that the annealshould be for a period of about two hours as the maximum improvement increep-rupture properties is then obtained, with little or no improvementresulting upon heating for longer times. It is found that, despite thehigh annealing temperature employed as aforedescribed, the alloy resistsgrain growth. Advantageously, the metal is rapidly cooled after theanneal, e.g., by water quenching or cooling in air.

In order to give those skilled in the art a better understanding of theadvantages of the invention, the following illustrative examples anddata are set forth.

A number of commercial scale melts were prepared in an arc furnace usingconventional air melting practice to provide alloys having thecompositions set forth in Table I hereinafter. In each instance, meltshaving the specified contents of nickel, chromium, iron, carbon andincidental elements was prepared. Shortly before casting the moltenbath, an amount of aluminum less than about 1% by weight of the bath wasintroduced therein, whereupon the requisite titanium addition was madeand the molten metal thus treated was cast into ingot molds.

TAB LE I Percent Alloy No. 0 Mn Fe S Si Cu Ni Cr Al Ti 0.40 0.83 48.000. 007 0.45 0.22 30.11 19. 96 O. 63 0. 51 0.44 0. 85 44. 63 0. 007 0.470.27 32. 64 20. 67 0.38 0. 50 0. 47 0. 75 46. (i2 0. 007 0. 30 0. 24 32.27 19. 23 0. 42 0. 52 0. 40 0.87 44. 94 0. 007 o. 56 o. 213 31.10 21. 870. 43 0. 58 0. 40 0. 70 45. 73 0. 007 0. 42 0. 24 31. 47 20. 02 0. 43 0.U0 0. 41 r). so 46. 0. 007 0. 3!) 0. 23 31. 50 20. 40 (1. 53 0. 64 0.3f) 0. 73 45. E17 0. 007 0. 38 0. 30 31. 78 20. 42 0. 38 0. 55 0. 41 0.60 44. 43 0. 0 0. 34 0. 24 33. TU 20. 16 0. 0. 52 0.34 0. 80 45. 3!) 0.007 0. 34 0. 41 33. 21 10. 48 0. 57 0. 53 0. 39 O. 83 44. 80 0. 007 0.38 0. 32 87 1!). 38 0 0. 52 0. 41 0. 70 45. 55 0. 007 0. 3S 0. 41 32. 5210.111 0. 52 0. 55 0. 41 0. 84 45. 25 0. 007 0.33 O. 00 31. 00 21. JO 0.57 0. 55 0. 0. 79 44. 11 0. 007 0. 37 0. 36 33. 04 10. 33 0. 50 0. 530.42 0. 81 44. 29 0.007 0.41 0. 38 32.54 19. 80 0. 5S 0 52 NorE.Thealloys were malleable over temperature ranges or about 1,700 F. to about2,300 F. as determined by usual production control tests. The alloyscontained molybdenum in amounts up to a nut 0.26% and not more than0.015% phosphorus.

Big ingots produced from the alloy may be converted to common mill formsby conventional operations, including hot rolling, forging, extrusion,cold rolling, etc., with usual mill process anneals at temperatures ofthe order of 1900 F. to about 2100 F. as required consistent with goodmill practice.

In order to contribute high creep-rupture resistance to the alloy, wefind that a heating in the temperature A portion of metal from Alloy No.1 was converted into extruded tube having an outside diameter of 6inches and a Wall thickness of one-half inch. Portions of the tube weresubjected to annealing treatments at various temperatures from 2150 F.to 235 0 F. and water quenched. Test specimens of the thus-treated metalwere subjected to stress-rupture testing at 1600 F. and 12,000 p.s.i.stress with the results set forth in the following Table II.

TABLE II Min. creep Annealing rate, Rupture life, Elongation, Averagegrain temp, F. Time, hrs. percent/hr. hrs percent RA. percent size, in.

range of about 2300 F. to about 2350 F. is necessary. The results setforth in the foregoing Table II demonstrate We find that the annealingtemperature should be at least about 2300 F. or the high level ofcreep-rupture properties is not obtained but that the annealingtemperature should not exceed about 2350 F. as otherwise the possibilityexists that incipient melting may be en- The alloy contemplated inaccordance with the invention becomes harder and stronger when aged inthe temperature range of about 1200 F. to about 1600 F. It is found,however, that prolonged heating of the alloy in the temperature range inwhich aging takes place does not result in any embrittlement as revealedby short-time tensile tests and by the Charpy V-Not-ch impact test.Thus, portions of 6-inch diameter extruded tube having a one-half inchwall produced from Alloy No. 1 were annealed for two hours at 2325 F.and water quenched. Test speciments of the material were subjected toshort-time tensile tests at various temperatures with the results setforth in the following Table III.

TABLE III Yield strength Tensile strength, Elongation, Tcn1p., F. (0.2%ofiset), p.s.i. p.s.i. percent Material of similar origin to thatreported in Table III was subjected to heating at 1400 F. for 1000hours. Test Specimens of this material were subjected to short-timetensile testing with the results set forth in the following Table IV.

It is to be noted that the 1400 F. ductility trough found for theas-annealed material was removed by the long-time exposure to 1400 F.The Charpy V-Notch impact strength of the material in the as-annealedcondition was 30 foot-pounds. It was found that the long-time exposureat 1400 F. for 1000 hours had little effect upon the impact toughness,since the impact value after the exposure was 26 foot-pounds.

Hot rolled rod material from Alloy No. 1 was annealed at 2300 F. for onehour and water quenched. Rotating beam fatigue data were obtained uponthis material with the results set forth in the following Table V.

TABLE V Fatigue strength, p.s.i.

10 cycles cycles 10' cycles 10 cycles TABLE VI Parameter (P), 1% plasticstrain Parameter (P) rupture Stress p.s.i.

For the parameter plot, the temperature-parameter abscissa relationshipis set forth in the following Table VII.

TABLE VII Parameter Parameter Parameter Parameter for for 1,000 for10,000 for 100,000 Temp, F. hour life hour life hour lite lieu; life Itwas found that an aging treatment for 8 hours at 1600 F. increased the1% strain parameter at 1800" F. and 4,000 p.s.i. from 38.7 to 39.5.

While the mechanism involved in providing the high creep-rupturestrength found in the alloy of the invention as a result of annealing attemperatures circa 325 F. is not fully understood, X-ray diffractionstudies of residues extracted from a slab forging of Alloy No. 3indicated that the basic carbide type of the as-forged material was M Cand no titanium carbide was detected. However, after the material hadbeen annealed at 2300 F. for 10 hours, the basic carbide type was Mqc3and there was definite evidence of titanium carbide. X-ray diffractionanalysis of residues from annealed material which had been exposed totemperatures on the order of 1400 F. indicated that a precipitation of M0 type carbides in a fine dispersion in the matrix had occurred.Regardless of the actual mechanism involved in achieving the highcreep-rupture strength through the high temperature annealing techniquein alloys of the invention, it is still found that surprisingly highcreep-rupture strength is developed in the alloys, although hardening bygamma prime precipitation apparently does not take place. It appearsthat the strengthening mechanism involving carbides which operates toprovide the high creep-rupture properties developed in the specialnickel-chromium-iron alloys provided in accordance with the invention isunique thereto and is not obtained in other matrix compositions.Furthermore, with the ability to work the alloys in conventionalequipment as is the case, and with the fact that none of the expensive,more exotic alloying ingredients is required, the basic cost of thealloy in wrought forms capable of industrial application is low incomparison to other metallic materials which have a similar strengthcapability. The alloys are weldable by the inert-gas shielded processusing either tungsten-arc or metal-arc procedures. Filler wire ofmatching composition is employed. Best results are obtained in weldingannealed material.

The alloy resists scaling upon exposure to heat under oxidizingconditions, resists sulfidation and other corrosive conditions andresists carburization at elevated temperatures. These properties,together with the high stressrupture properties of the alloy, make itadvantageous in many applications, including furnace equipment, baskets,trays, muffies, radiant tubes, etc, in the petrochemical field forreformer and cracker tubes, hot die platens and many others.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. A nickel-chromium-iron alloy consisting essentially of about 29% toabout 40% nickel, about 19% to about 25% chromium, about 0.2% to about0.5% carbon, about 7 0.25% to about 1.25% titanium, up to about 1%aluminum, up to about 0.75% silicon, up to about 1.5% manganese, and thebalance, including small amounts of incidental elements and impurities,being essentially iron.

2. An alloy according to claim 1 wherein the nickel content is about 30%to about 35%, the chromium content is about 19% to about 23%, and thetitanium content is about 0.35% to about 1%.

3. An all-y according to claim 1 in the condition resulting from aheating in the temperature range of at least about 2300 F. to about 2350F. for at least about two hours, whereby the resistance of the alloy tocreep and rupture is greatly increased.

4. An alloy according to claim 1 having a microstructure characterizedby the presence of titanium carhide and of carbides having the typesM7C3 and M C 5. The method for producing improved creep-rupture strengthin alloy consisting essentially of 29% to 40% nickel, about 19% tochromium, 0.2% to 0.5% carbon, 0.25 to 1.25% titanium, and the balanceessentially iron, which comprises annealing a wrought article made ofsaid alloy at a temperature of 2300 F. to 2350 F. for at least twohours.

6. The method according to claim 5 wherein the alloy contains 30% tonickel, 19% to 23% chromium and 0.35% to 1% titanium.

7. An alloy consisting essentially of about 0.34% to about 0.47% carbon,about 30.11% to about 33.87% nickel, about 19.23% to about 21.90%chromium, about References Cited UNITED STATES PATENTS 2,597,173 5/1952Patterson -128.8 X 2,606,113 8/1952 Payson 148136 X 2,661,284 12/1953Nisbet 148l36 X 2,686,116 8/1954 Schempp 148-136 X 2,813,788 11/1957Skinner 148128.8 2,879,194 3/1959 Eichelberger 75-128.8 3,184,577 5/1965Witherell 75--128.8

OTHER REFERENCES Delta Ferrite Formation and Its Influence on theFormation of Sigma in a Wrought Heat Resisting Steel, pp. 11-12,preprint 1948 by Gilman et 211., published by American Society forMetals.

HYLAND BIZOT, Primary Examiner US. Cl. X.R. 148-136 mg UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,459 539 DatedwInventofls) Herbert L. Eisels a I It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 2, line 36, for "10%" read --l9%--.

Column 5, Table III, second column, for "26,000" read R ---26,500-. SameTable, same column, for "24,000" read Column 5, Table IV, last column,last number, for "6.0" read Column 7, Claim 5, line 2, before "alloy"insert --an--.

Same Claim, line 3, delete "about" Column 8, Claim 8, line 1, before"chromium" insert the-.

Signed and sealed this 11 th dag, of May 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SGHUYLER, JR. Attesting OfficerCommissioner of Patents

